1. Field of the Invention
The invention relates to a method and apparatus for carrier frequency offset detection in an Orthogonal Frequency Division Multiplexing (OFDM) system, and more particularly, to a method and apparatus for using a hierarchically modulated data signal to detect carrier frequency offset.
2. Description of the Related Art
Recently, Orthogonal Frequency Division Multiplexing (OFDM) has become an important wireless communication technology. Because of the high transmission rate of OFDM technology offers high transmission rates, data can be easily and efficiently transmitted and received in a wireless communication environment. Therefore, OFDM technology has been widely applied to, for example, Digital Audio Broadcasting (DAB), Digital Video Broadcasting-Terrestrial/Handheld (DVB-T/H), Wireless Fidelity (Wi-Fi) and Worldwide Interoperability for Microwave Access (WiMAX) etc., and is also regarded as a fundamental and core technology in the 4th-Generation Wireless System.
For OFDM technology, data is transmitted by a plurality of subcarriers which overlap and are orthogonal with each other. In addition, duplicated data which is copied from the end of a portion of a symbol is defined as a cyclic prefix (CP) or a guard interval (GI), and is used to protect the OFDM symbol from inter-symbol interference (ISI) generated by multi-path fading and reflection in channels. The bandwidth used by the OFDM system is divided into a number of narrow sub-bands so that the sub-bands are only affected by flat fading. Thus, only one standard equalizer is needed in the receiver to adjust signal gain and compensate for flat fading of channels. In this fashion, the OFDM system has many advantages such as having multi-path fade resistance properties, high-efficiency bandwidths, low-complexity equalizers and high transmission rates . . . etc.
However, the OFDM system is affected by Doppler effect under an environment of high speed movement, such as that which may be found with high speed rail trains. The OFDM system which is regarded as a multi-carrier system is very sensitive to carrier frequency offset (CFO) caused by the Doppler effect. Carrier frequency offset will destroy the orthogonality between subcarriers and generate inter-carrier interference (ICI) between the subcarriers such that performance and bit error rate of the OFDM system in a high speed environment decreases and increases, respectively. Therefore, an important issue to be solved in the OFDM system, is, efficient detection of carrier frequency offset (CFO) to cancel inter-carrier interference (ICI) between subcarriers.